In many industrial and commercial operations, large wheeled carts and the like are used to transport parts, components and similar items from one location to another. For example, in the automotive industry, it is common practice to use such wheeled carts to transport parts from place to place in assembly line operations, and in many instances, these carts are quite large so as to be capable of transporting parts and components that are both large and heavy, and they must be rolled over floor surfaces and terrains that are rough and uneven. As a result, these carts and the cargo carried therein are subjected to jolts and bounces during movement across the uneven surface, which can result in undesirable consequences, including damage to the cargo and the generation of noise levels that may exceed acceptable limits such as those determined by OSHA.
In an effort to deal with these undesirable consequences, these wheeled carts are usually provided with specially formed wheels designed to absorb the shock loads imposed on the cart during its movement over uneven or rough surfaces. A typical example of such wheels is one which includes a cart iron core or annular rim member formed at the exterior or peripheral surface thereof with a one-half inch to one inch layer of molded polyurethane. However, while these molded polyurethane coverings do tend to absorb some of the shock loads imposed on the cart, they generally do not absorb enough of the shock load in some heavy-duty cart applications.
It has also been heretofore proposed that the wheel assembly be formed with a polyurethane covering at the exterior surface of the cast iron core, and with an additional and separate annular layer of polyurethane disposed between the interior annular surface of the core and the exterior annular surface of the bearing seat member of the wheel. Wheels of this type perform better in terms of absorbing shock loads, but they tend to be difficult to manufacture and somewhat unreliable because the intermediate layer of polyurethane must be poured into the annular spacing between the core and the bearing seat, and, during curing of the polyurethane, there is an inherent shrinkage of the polyurethane, which can cause the outer surface of the polyurethane to be drawn away and separated from the inner surface of the core, whereby the bearing seat and the intermediate layer of polyurethane can become separated from the core. The tendency of the polyurethane to shrink can be offset to a large extent by providing for controlled cooling of the polyurethane, during curing, over a long period of time under carefully controlled conditions, but this process substantially increases the cost of manufacture of the wheels, even to the point, in some cases, of making the process commercially prohibitive.
By contrast, the present invention comprises a shock-absorbing wheel assembly which provides at least the same capacity to absorb shock loads as that of the double-layer polyurethane which is described above, yet it can be produced at significantly reduced manufacturing costs and it has good structural integrity.